A base station which carries out wireless communication with plural mobile stations, for example, assigns a radio resource, in which data transmission is permitted, on the time basis in response to a request for wireless transmission from a mobile station so that no interference occurs between wireless signals transmitted wirelessly from the plural mobile stations.
In the Long Team Evolution (LTE) of the 3rd Generation Partnership Project (3GPP), 12 subcarriers transmitted in a wireless zone are handled as a single resource block (RB). The base station assigns a communication frequency band on the RB basis for each mobile station. The frequency band assigned on the RB basis is a radio resource of each mobile station.
Each mobile station transmits data to the base station using the assigned radio resource. The base station decodes the received data and carries out an error correction decoding process to the decoded data. If the decoding process has successfully been completed, the base station transmits an acknowledgement (ACK) signal to the mobile station which has transmitted the data. The ACK signal is a reception acknowledgement signal used for the acknowledgement of reception of the data. If the decoding process has not been successfully completed, a negative acknowledgement (NACK) signal is transmitted to the mobile station which has transmitted the data.
The mobile station which received the ACK signal temporarily stops the transmission process, regarding that the transmission data has been successfully received by the base station. The mobile station which received the NACK signal transmits the same data again to the base station using a radio resource determined in accordance with a rule that the base station and the mobile station already know, regarding that the transmission data has not been successfully received by the base station. The mobile station repeats transmission of the same data until the mobile station receives the ACK signal.
In a case in which the base station transmitted the ACK signal but the mobile station has not successfully received the ACK signal due to, for example, an environmental influence of the wireless communication, the mobile station carries out a retransmission process of the data regarding that the mobile station has received a NACK signal. Repeated transmission of the same data decreases system throughput. For example, Japanese Laid-open Patent Publication No. 2009-188660 describes a technique to reduce the frequency of occurrence of retransmission by reducing occurrence of erroneous determination regarding the ACK signal which has been transmitted to the mobile station from the base station.